1. Field
The present invention relates generally to communication systems, and more specifically, to a method and an apparatus for selecting a serving sector in a data communication system.
2. Background
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting information signals from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication channel bandwidth. At the destination station the original information signal is replicated from the modulated carrier wave received over the communication channel. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of remote subscriber units requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), and amplitude modulation multiple-access (AM). Another type of a multiple-access technique is a code division multiple-access (CDMA) spread spectrum system that conforms to the xe2x80x9cTIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,xe2x80x9d hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention.
A multiple-access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSIONxe2x80x9d, assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of a communication systems carrying both voice and data comprise communication systems conforming to the xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or xe2x80x9cTR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systemsxe2x80x9d (the IS-2000 standard).
In a multiple-access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one mobile subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.
An example of a data only communication system is a high data rate (HDR) communication system that conforms to the TIA/EIA/iS-856 industry standard, hereinafter referred to as the IS-856 standard. This HDR system is based on a communication system disclosed in co-pending application Ser. No. 08/963,386, entitled xe2x80x9cMETHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,xe2x80x9d filed Nov. 3, 1997, assigned to the assignee of the present invention. The HDR communication system defines a set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which an access point (AP) may send data to a subscriber station (access terminal, AT). Because the AP is analogous to a base station, the terminology with respect to cells and sectors is the same as with respect to voice systems.
A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 ms. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques which require significantly larger delays than those that can be tolerated by voice services can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. patent application Ser. No. 08/743,688, entitled xe2x80x9cSOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDS,xe2x80x9d filed Nov. 6, 1996, now U.S. Pat. No. 5,933,462, issued Aug. 3, 1999, assigned to the assignee of the present invention.
Another significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal transmission rate for all users and a maximum tolerable value for the error rates of the speech frames. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system is typically defined as the total delay incurred in the transfer of a predetermined amount of data, hereinafter referred to as a data packet.
Yet another significant difference between voice services and data services is that the former requires a reliable communication link. When a mobile station, communicating with a first base station, moves to the edge of the associated cell or sector, the mobile station initiates a simultaneous communication with a second base station. This simultaneous communication, when the mobile station receives a signal carrying equivalent information from two base stations, termed soft handoff, is a process of establishing a communication link with the second base station while maintaining a communication link with the first base station. When the mobile station eventually leaves the cell or sector associated with the first base station and breaks the communication link with the first base station, it continues the communication on the communication link established with the second base station. Because the soft handoff is a xe2x80x9cmake before breakxe2x80x9d mechanism, the soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a mobile station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled xe2x80x9cMOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,xe2x80x9d assigned to the assignee of the present invention. Softer handoff is the process whereby the communication occurs over multiple sectors that are serviced by the same base station. The process of softer handoff is described in detail in co-pending U.S. patent application Ser. No. 08/763,498, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION,xe2x80x9d filed Dec. 11, 1996, now U.S. Pat. No. 5,933,787, issued Aug. 3, 1999, assigned to the assignee of the present invention. Thus, both soft and softer handoff for voice services result in redundant transmissions from two or more base stations to improve reliability.
This additional reliability is not required for data transmission because the data packets received in error can be retransmitted. For data services, the parameters, which measure the quality and effectiveness of a data communication system, are the transmission delay required to transfer a data packet and the average throughput rate of the system. Transmission delay does not have the same impact in data communication as in voice communication, but the transmission delay is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system. Consequently, the transmit power and resources used to support soft handoff can be more efficiently used for transmission of additional data. To maximize the throughput, the transmitting sector should be chosen in a way that maximizes the forward link throughput as perceived by the Access Terminal (AT).
There is, therefore, a need in the art for a method and an apparatus for selecting a sector in a data communication system that maximizes the forward link throughput as perceived by the AT.
In one aspect of the invention, the above-stated needs are addressed by determining at the remote station a quality metric of a forward link for each sector in the remote station""s list, determining a quality metric of a reverse link to each sector in the remote station""s list, and directing communication between the remote station and one sector from the sectors in the remote station""s list in accordance with said determined quality metric of a forward link and said determined quality metric of a reverse link. The quality metric of a forward link for each sector in the remote station""s list may be determined by measuring a signal-to interference and signal-to-noise ratio of the forward link. The quality metric of a reverse link to each sector in the remote station""s list may be determined by processing at the remote station the forward link from each sector in the remote station""s list. The signal processed may be obtained by measuring at each sector the quality metric of the reverse link, processing the quality metric to provide an indicator of the quality metric, and providing the indicator on a forward link. The communication between the remote station and one sector from the sectors in the remote station""s list may be directed in accordance with said determined quality metric of a forward link and said determined quality metric of a reverse link by assigning credits to each sector in the remote station""s list, except a sector currently serving the remote station in accordance with said determined quality metric of a forward link and said determined quality metric of the reverse link, and directing communication between the remote station and one sector from the sectors in the remote station""s list in accordance with said assigned credits.
In another aspect of the invention, the above-stated needs are addressed by determining at the remote station a quality metric of a forward link for each sector in the remote station""s list; and directing communication between the remote station and one sector from the sectors in the remote station""s list in accordance with said determined quality metric of a forward link. The quality metric of a forward link for each sector in the remote station""s list may be determined by measuring a signal-to-interference and signal-to-noise ratio of the forward link. The communication between the remote station and one sector from the sectors in the remote station""s list may be directed in accordance with said determined quality metric of a forward link by assigning credits to each sector in the remote station""s list, except a sector currently serving the remote station in accordance with said determined quality metric of a forward link and directing communication between the remote station and one sector from the sectors in the remote station""s list in accordance with said assigned credits.